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Research Document - 2007/064

Determination of Geoduck Harvest Rates Using Age-structured Projection Modelling

By Zhang, Z. and C. Hand


An age-structured projection model was used to study the impacts of alternative exploitation intensities on geoduck populations, based on a large accumulation of survey data, age composition data and published estimates of natural mortality. Historic recruitment patterns were back-calculated using an age-structured model and future recruitment was simulated from the historic pattern. The fishing patterns examined were combinations of different instantaneous mortality rates (0.016 or 0.036) and different periods of historical recruitment (beginning in 1940 or 1960). For each simulation year, in each set of 1000 runs, the ratio of current biomass to virgin biomass was calculated and compared to the management objective of not allowing the biomass to fall below 50% of the virgin biomass within 50 years of initial harvest on a bed. Data were analyzed and results presented by geographic region.

Trends in recruitment were independent of the value of M used, although rates were higher when M of 0.036 was applied compared to M of 0.016. Historic recruitment rates were found to be highly variable in the pre-fishery state: rates increased from the early 1930’s to 1950, decreased until early 1960, increased to another peak in the mid 1960’s declined until the mid 1980’s and have been increasing to present.

A precautionary and reasonable scheme appears to be Y40M0.036 (recruitment simulation year starting in 1940 and M 0.036) for the north coast, and Y60M0.016 for the central coast and Queen Charlotte Islands. For the west coast of Vancouver Island and Georgia Basin, Y60M0.036 is precautionary. Overall, an exploitation rate of 1.2% and 1.8% of estimated current biomass is recommended for the west coast of Vancouver Island and the rest of the coast respectively. Further suggestions for managers are made on limit reference points and the choice of exploitation rate from the range provided.

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